Any vitamin or mineral taken in excess could interfere with the absorption, metabolism or storage of some other component of the diet, but interactions are usually not a problem.
Nutritionists basically recommend that people should derive their nutrition from food, not food supplements, and that they should choose a well-balanced diet including a variety of fruits and vegetables.
Difficulties in the interaction of dietary supplements come only when people take unreasonable amounts.
In a nutritious meal, vitamin and mineral interaction is minimal, and normal-strength multivitamin supplements should present no problem.
It is true that certain nutrients do interact if they get into the body in very unbalanced amounts.
For example, iron does compete with the absorption of zinc, and the therapeutic amount of iron for someone with anemia might lead to an imbalance, but the amounts in a normal diet would not create difficulties.
Vitamin and mineral interactions are quite different from the dangerous interaction of one drug with another, like alcohol with barbiturates, and it is mostly long-term dietary imbalances that would cause trouble.
Nutritionists suggest staying away from high-potency supplements, especially of just one or two nutrients, unless a doctor suggests a specific one.
Anything in excess, if not balanced by other nutrients, could result in a deficit of something else.
As for combinations, again it is the total dietary picture that counts.
For example, vitamin D does enhance calcium absorption, but it is not necessary to take the vitamin and the mineral at the same time.
